because it is harming bitcoin. you don't see people go around and say how they hate XYZ coin when they get pumped! that is because any other altcoin getting pumped just means that altcoin getting pumped and some traders shift and enjoy the profit.
but when bitcoin cash gets pumped that means hashrate travels from bitcoin to bitcoin cash and when hashrate drops on bitcoin network it leads to slower network so to speak. blocks are found slower and that leads to this huge back log and then the higher fees.

all damages caused by bitcoin cash. you can see be fore the BCH pump bitcoin was fine, no back log, empty mempool and extremely low fees!